avformat: add vapoursynth wrapper

This can "demux" .vpy files. Autodetection of .vpy scripts is intentionally not done, because it would be a major security issue. You need to force the format, for example with "-f vapoursynth" for the FFmpeg CLI tools.

Some minor code copied from other LGPL parts of FFmpeg.

I did not find a good way to test a few of the more obscure VS features, like VFR nodes, compat pixel formats, or nodes with dynamic size/format changes. These can be easily implemented on demand.